I. Field of the Invention
This invention relates generally to bone screws and more particularly, in some respects, to bone screws comprising a dilation between the head and the threaded portion that may be configured to be coupled to a crosslink.
II. Description of Related Art
Bone screws are used in numerous surgical operations. In particular, polyaxial bone screws are often used in spinal surgery. Known polyaxial bone screws have a spherical, semi-spherical, or otherwise rounded head, and a cap known sometimes as the “tulip,” with the cap and head cooperating to form what is sometimes referred to as the “polyaxial head.” A set screw is used to lock a structure such as a rod to the polyaxial head by being threadably engaged with the tulip. The cap may include a socket portion capable of receiving the bone screw at multiple orientations, much like a ball-and-socket joint. This allows some flexibility for the surgeon placing the screws because the orientation of the screw does not typically limit the orientation of the member attached to it.
However, in certain instances, the bone screw is screwed in such that the rounded head becomes incapable of being polyaxially coupled to the cap. For example, the bone screw may be screwed in too far and at such an angle that the cap is not free to move polyaxially with respect to the head of the screw once the head impinges on the surface of the bone. When this happens, the bone screw should not be used to bring two boney surfaces together. The phrase “lag effect” is commonly used to describe what happens when two fractured bones or the surfaces of two joints are compressed by the force of the screw threads in the more distal bone pulling the head of a single screw against the proximal bone; a bone screw in that position is commonly described as a “lag screw.” Traditional lag screws have conical heads that lack any intrinsic polyaxial ability and therefore are difficult to incorporate into a construct that involves multiple screws at varying angles. In contrast, screws with polyaxial heads can more easily be incorporated in long constructs with multiple screws of varying angles, as seen in the spine. However, such polyaxial screws are only able to provide compression by placing two separate screws (one in each bone or joint surface), connecting them with a rod, and then applying the compression through the rod.
Multiscrew constructs, in the spine for instance, often incorporate linkages between the screws placed on the right and left sides of the spine to increase stability. Such a “crosslink” is usually done between two screws from rod to rod, or it can be done from the top of one polyaxial head to another. Placing the crosslink on the rods can sometimes be difficult or impossible because there is often little space between the screw heads on the rod to place the crosslink. When the crosslink is attached to the top of a polyaxial screw, sometimes the angulation of the two screws is very different and it is difficult or impossible to attached a rigid crosslink that matches those angles.